Unlike open flue wood or gas fireplaces that draw combustion air from the room in which they are located, the fireboxes of direct vent gas fireplaces are sealed from the room air. The fireplaces draw combustion air into the firebox from outside the building through air intake ducts and exhaust the combustion gases from the firebox out of the building through exhaust ducts. Air flow through the firebox and the ducting systems during operation of direct vent gas fireplaces is typically driven by thermal convection and the buoyancy of the combustion products. Relatively cool combustion air is drawn in and down a vertically offset run of inlet duct while heated air rises along a vertically offset run of the exhaust duct. Ideally, the intake and exhaust ducts are entirely vertical, but they may vent horizontally from a wall of the building provided there is an adequate vertical run to ensure gravity-fed operation.
As discussed in U.S. Pat. No. 5,267,552 to Squires et. al, air flow through the firebox and the duct system may be enhanced by a blower. This can be particularly useful when the ducts are not sufficiently vertical to rely on convection and the buoyancy of the gas combustion products.
One problem in the management of air flow of direct vent gas fireplaces is to ensure that a sufficient amount of combustion air is available during ignition of the fireplace while avoiding inefficiencies due to the loss of heated combustion products during operation of the fireplace. Optimizing the efficient operation of the fireplace entails restricting the excess air flow through the firebox and to the combustion gas outlet while the fireplace is in operation. However, such restriction poses a corresponding problem during start-up when the air in the intake and exhaust vents is cool and stagnant. In order achieve effective ignition of the burners, the air flow must be as unrestricted as possible so as to enable it to immediately begin moving. The failure to do so may result in lift-off of the flame and the ignition of pockets of built up gas, or in starving the ignition system of combustion air and a failure to maintain ignition.
The optimal air flow required for successful ignition is therefore greater than the air flow desired for maximum efficiency during operation of the fireplace. This trade off is usually addressed by, at the time of installation, selecting a degree of restriction for the fireplace that achieves a measure of efficiency during fireplace operation but that also provides for sufficiently unrestricted movement of air during start up. The solution is nonetheless inefficient.
European Patent App. No. 0268407 to Shimek et al. discloses a direct vent gas fireplace in which a primary air supply is provided through a primary air duct. However, when the combustion chamber is cold, there is not a sufficient supply of hot exhaust gases to produce or induce a draw from the primary air duct. A secondary slot is therefore provided to allow entry of a supply of secondary air directly to the burner area to assist in igniting the burner under cold conditions. This secondary slot is closed off by a pivoting damper, actuated by a bi-metallic element, once the fireplace has heated up sufficiently. While this approach may assist in maintaining ignition, it still suffers the drawback of significant heat losses during continued operation of the fireplace because it is not intended to control overall air flow through the fireplace.
In sealed combustion gas fireplace systems, the problem of ignition air volume is sometimes addressed by maintaining a standing pilot flame that generates a small amount of air flow through the fire box and venting system. This is particularly so in colder climates. However in some jurisdictions, the use of a standing pilot flame is falling out of favour with regulatory authorities because of perceived energy conservation reasons. As the standing pilot has almost universally been used to ensure air flow through the firebox and duct system in colder climates, the absence of a standing pilot creates a challenge to reliable ignition of the burner.
U.S. Pat. No. 5,503,550 to DePalma discloses a gas fireplace system in which a manually operated pilot light unit replaces the standing pilot light. An automatic damper mechanism, including a rotatable damper vane and a controller actuated by a motor, controls air flow through the flue in the fireplace. The controller is hard-wired into the electric lines of the building in which it is installed, which may make the system difficult to install or retrofit. In an alternative embodiment, the damper vane includes thermally-activated bi-metallic quadrants. However, the quadrants are designed to keep the flue closed until the fireplace produces enough heat that the vanes to flex to an open flue position. This does not assist in ensuring that there is a sufficient air supply at ignition of the fireplace burners.
U.S. Pat. No. 7,451,759 to Weiss et al. discloses a wireless damper control device for a fireplace, eliminating the need to hard-wire a control system into the building. This damper control system will open the damper immediately upon request to ignite the gas fireplace. Once the damper indicates to the control system that it is open, gas is allowed to flow to the ignition system. This should allow enough air to flow to successfully ignite the fireplace, but the circuitry and signalling required in this system is relatively complex. Further, in this system, the damper remains open during the burner operation and is only intended to close the flue system in the off cycle.
It is therefore an object of the present invention to provide for the efficient control of air flow in a direct vent sealed combustion gas fireplace that does not have a standing pilot, that maximizes the operating efficiency, while enabling sufficient air flow during a cold start to maximize the chances of successful ignition without the need for complicated electronics or signalling mechanisms.
This and other objects of the invention will be better understood by reference to the detailed description of the preferred embodiment which follows. Note that not all of the objects are necessarily met by all embodiments of the invention described below or by the invention defined by each of the claims.